I am serioulsy considering buying a new i7 mac mini - so is it worth waiting for the new 2013 version or not ? in other words how can they improve on what already sems a very fast spec apart from speed and size of the hdd ?

It appears to be a 2TB 2.5" drive. News to me. I thought 1.5TB was the largest currently offered in a 2.5" form factor.

I looked up the specs on Western Digital's site and it's a 15mm drive height. That is hugely tall. People have had success getting 12.5mm drives into Mac minis with modifications... so perhaps where there is a will there is a way. The mini is designed to support a 2.5" drive with a 9.5mm drive height.

If you really need the storage space and it needs to be internal you can always get a mini with 2 hard drives inside it. ie. the server model or buld your own with 2 drives.